The influence of upper airway patency on ventilation assisted by chest negative pressure ventilation (CNPV) or nasal intermittent positive pressure ventilation (nIPPV) was studied as follows. 1) In seven patients with chronic respiratory failure (PaCO2 more than 50 Torr), the increase in tidal volume (VT) induced by CNPV was larger during mouth breathing than during nose breathing in the awake state. On CNPV transcutaneous PCO2 (PtcCO2) decreased during awake state, but increased during NREM sleep. 2) In four patients with chronic respiratory failure (PaCO2 more than 60 Torr), nIPPV induced the leakage of air from mouth in more than 20 cmH2O of nasal mask pressure during sleep. PtcCO2 increased during sleep, especially during REM sleep in spite of nIPPV. The change in PtcCO2 during REM sleep on nIPPV comparing awake state was 16.1 +/- 1.4 torr and comparing REM sleep in usual sleep was -6.0 +/- 1.4 Torr. 3) Upper airway resistance (UAR) was measured in two patients with tracheostomy. An increase in UAR was associated with a linear decrease in VT during nIPPV, although associated with a curvilinear decrease in VT during CNPV. These results indicate that the efficiency of CNPV and nIPPV depends on the patency of upper airway.